; Exercise 1.21
; Use the smallest-divisor procedure to find the smallest divisor of each of 
; the following numbers: 199, 1999, 19999.
(define (smallest-divisor n)
  (find-divisor n 2))
(define (find-divisor n test-divisor)
  (cond ((> (square test-divisor) n) n)
        ((divides? test-divisor n) test-divisor)
        (else (find-divisor n (+ test-divisor 1)))))
(define (divides? a b)
  (= (remainder b a) 0))
(define (prime? n)
  (= n (smallest-divisor n)))

(prime? 199) ;#t
(prime? 1999) ;#t
(prime? 19999) ;#f

